An Australian vernacular phrase (possibly also found elsewhere in the Commonwealth) meaning "full to the brim" or "jam-packed". Alternatively spelled chocka-block, chock-a-block, etc. Synonymous with "chock-full".

For example: "I had a big night out bending the elbow and I had to find somewhere to chunder, but the dunnies were chock-a-block so I had to use some sheila's handbag!"

DonJaime says this phrase is also very common in the UK.
locke baron says it is used in the US, too, though usually in the form "chockablock full". May be a New England thing.